We developed a decision support system (DSS) for the aircraft maintenance department of KLM Royal Dutch Airlines at Schiphol Airport. This department inspects and maintains aircraft during their ground time at the airport. Its main resource is its workforce. Since January 1990 the DSS has supported management in analyzing several capacity planning problems related to the size and the organization of the workforce. In particular, management uses the DSS to determine the appropriate number of maintenance engineers and their training requirements, and to analyze the efficiency and effectiveness of the maintenance department.